Multiple channel fiber optic continuity test system

ABSTRACT

An optical system for individually testing each one of a plurality of fibers for continuity. Each fiber having a proximal end and a distal end with a dichroic reflector at each distal end reflective of wavelengths respective to a source of testing light and transmissive to other wavelengths. The selection of which fiber is to be tested is only a matter of manipulating a rotator element which has the capacity to rotate the angle of polarization by 90 degrees. The polarization of the light determines the respective fiber to be tested.

FIELD OF THE INVENTION

This invention relates to the testing of optical fibers for continuity.

BACKGROUND OF THE INVENTION

Optical fibers are commonly used to convey laser light for a widevariety of purposes. It is not infrequent for a single fiber with a corediameter on the order of 400 microns to be intended to carry laser lightpulses for actuation purposes. Examples are ordnance firing systems. Thefibers generally have a glass core with a glass cladding. They areterminated at a coupling to the user device. Depending on theconfiguration of the hardware, for example bulkheads and other placeswhere a discontinuity in the fiber is required, there will be additionalcouplings in the optical circuit.

The optical continuity of the fiber and of the joined fibers is ofutmost concern. The triggering of essential ordnance could be totallyfrustrated by a broken fiber or by a faulty connector. There are, ofcourse, fiber optic systems for other uses in which continuity is alsoof primary importance. In all of these it is common practice to conductfrequent reliability tests.

Conventional testing techniques utilize time-domain reflectometry (TDR).These systems operate by transmitting a short pulse of light through thefiber and detecting its reflection from a dichroic mirror at the end ofthe fiber or at the end of a series of fibers that are coupled together.The dichroic mirror is reflective to the frequency of the test pulse,but transmissive of the frequency of the light from a firing or a signalpulse. The short period of time it takes for the pulse to traverse thefiber in two directions is known, and the receiver will see two pulsesspaced a short time apart. The first pulse is light scattered from thetransmitted pulse. The second pulse is the reflection from the dichroicmirror at the end of the fiber. The pulses are fed into a high speedvoltage comparator which determines the pass/fail level. The pulses maybe logically separated from each other to detect only the reflectedpulse.

The TDR technique is used successfully for single fibers, one system foreach fiber. However, there are many installations in which two or morefibers are used. For example, some laser ordnance systems require theinitiation of simultaneous events. Commonly this is done with the use oftwo or more fibers, one respective to each event. While a test systemcan be provided for each of them, this soon becomes an economic burden,and in airborne systems, is an unacceptable weight penalty.

There are three common techniques for testing the continuity of systemswhich utilize more than one fiber. One technique is to require that bothor all fibers be the exact same length. Then the reflected pulses willsum together at the detector. Practical system variables such asconnector losses render this technique impractical. A reliable pass/faillevel cannot be determined.

A second technique is to require the fibers to be of sufficientlydifferent length so that separate pulse reflections can be detected foreach fiber. This seriously complicates the detection circuitry andplaces unnecessary constraints on system design.

A third technique is to use a different wavelength laser diode for eachfiber. This technique presents substantial laser and logistic problemsboth as to the electrical system and as to suitable dichroic mirrors forresponding to the multiple wavelengths.

However complicated and troublesome, these techniques have found activeuse, not because they are especially good, but because they have beenthe best state of the art. It is an object of this invention to providea continuity test system in which a single set-up can be used to test aplurality of optical fibers, thereby to reduce the cost and complexityof such systems, and to provide a more reliable pass/fail level.

The test system according to this invention utilizes polarized light andselector means respective to a plurality of cables, responsive topolarized light to select which of the fibers is to be tested forcontinuity. The selection requires only the manipulation of one opticalelement.

BRIEF DESCRIPTION OF THE INVENTION

A test system according to this invention utilizes a laser diode or someother source of pulsed light. It produces light which is non-polarizedor has a low polarization ratio. This light is incident on a polarizerwhich outputs light polarized in a reference plane, and passes otherlight out of the system. Polarized light from the polarizer impingedupon a non-polarizing first beam splitter is partly passed out of thesystem, and partly reflected along a detector axis aligned with adetector and with a rotator element which has the capacity to rotate theangle of polarization by 90 degrees relative to a reference axial plane.Inserting the rotator element in the path of the light, and removing it,rotates the polarization 90 degrees each time.

A polarizing second beam splitter receives the light from the first beamsplitter, directly or through the rotator, so as to produce either areflected beam or a transmitted beam, each having a respectivepolarization. A fiber to be tested for continuity is aligned andoptically coupled with a respective beam.

A dichroic mirror is placed at the distal end of each fiber, reflectiveto the test light wavelength and transmissive to operating wavelengths.Selection of which fiber is to be observed is only a matter ofmanipulating the rotator. The invention will be fully understood fromthe following detailed description and the accompanying drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a semi-schematic drawing showing the system of the inventionin one of its configurations, and

FIG. 2 is a drawing as in FIG. 1 showing the system in its otherconfiguration.

DETAILED DESCRIPTION OF THE INVENTION

The purpose of this invention is to test the conformity of a firstoptical fiber 10 and a second optical fiber 11. Dichroic reflectors 12,13 are placed adjacent to the distal ends 14, 15, of fibers 10, 11,respectively. The dichroic reflectors are reflective of the wavelengthsemitted by a laser diode 16 for test purposes, but are transmissive ofthe wavelengths of some other light source such as a different type oflaser.

A detector 18 responsive to wavelengths emitted by diode 16 sensesreturned light of those wavelengths, and provides an output signal tocircuitry (not shown) which determines whether the sensed light isreceived from a continuous fiber, or whether the fiber has adiscontinuity.

For convenience in disclosure the term "horizontal" and "vertical" willbe used herein to denote a pair of planes of polarization. These arearbitrarily selected terms, and have no necessary relationship to anyterrestrial references whatever. These are merely shorthand terms todenote two axially-oriented planes which form a 90 degree dihedralangle. In the drawings, the plane of the various light beam segments isregarded as the horizontal. Arrows 25 and others that are parallel tothem are parallel to or in that plane, and are denoted as horizontal.Arrows 26 and others that are parallel to them are perpendicular to thatplane, and are denoted as vertical.

The laser diode emits bursts of coherent light (incoherent light mayalso be used, although coherent light is preferred) which are not highlypolarized (sometimes called "randomly or eliptically polarized"). Arrows25 and 26 denote its vertical and horizontal components. Anypolarization may be defined by varying the relative amplitude of thesetwo components.

The configuration of FIG. 1 is set to test the continuity of fiber 10.Beam segment 27 extends from laser diode 16 to a first polarizer 28.This polarizer is shown schematically. Its property is to pass alllight, except for vertically polarized light, out beam segment 29. Itreflects vertically polarized light along beam segment 30. Light onsegment 29 is lost to the system. Beam segment 30 consists of reflectedvertically polarized light as shown by arrows 31. Element 28 may be anytype of high extinction ratio polarizer. Laser diode 16 may be orientedalong the axis of beam segment 30 provided polarizer 28 is adjusted toyield vertically polarized light along beam segment 30. If a laser diodeor other light source is used which produces a high polarization ratiothen the polarizer 28 is not required.

Beam segment 30 impinges on a non-polarizing second beam splitter 32,which transmits part of the light along beam 33, which is lost to thesystem, and reflects (at 90 degrees) the remainder of the light alongbeam segment 34. Light in segment 34 is vertically polarized as shown byarrows 35.

In this configuration, beam segment 34 impinges on half-wave plate 36whose property is to rotate the polarization by twice the angle betweenthe input polarization and the optic axis 60. Since the optic axis 60 ispositioned at a 45 degree angle relative to the input polarization, theoutput polarization is rotated by 90 degrees. Thus, the plane ofpolarization in beam segment 36a is now horizontal as shown by arrows37. Light in segment 36a impinges on a total reflector 38 such as amirror or totally reflecting prism. In turn, beam segment 39 isreflected from the reflector, remaining horizontally polarized as shownby arrows 40. Reflector 38 is provided as a means to enable the testequipment to be placed to one side of a system. Should this not bedesired or necessary, reflector 38 can be eliminated and the systemdesigned without the offset it provides.

Beam segment 39 impinges on a polarizing beam splitter 41 whose propertyis to pass horizontally polarized light along beam segment 42, as shownby arrows 43. It reflects vertically polarized light at a 90 degreeangle, but in this system configuraton there is none to reflect, and sono light from the laser diode 16 is reflected toward second fiber 11.

A focusing lens 44 focuses light from segment 42 onto the end of fiber10, which passes through the fiber to dichroic reflector 12, and isreflected by it back to lens 44 if there are no discontinuities in thefiber. The returning light on segment 42 is randomly polarized, meaningthat it is not polarized in a single plane. This is due to the fact thatthe fiber is multi-mode. Returning along segment 42, it impinges onpolarizing beam splitter 41, which passes only horizontally polarizedlight. The other light is wasted from the system by reflection alongsegment 47. The returning light along segment 39 is again horizontallypolarized as shown by arrows 40. It is reflected by reflector 38, andimpinges on rotator 36, which again rotates the polarization 90 degreesto a vertical plane as shown by arrows 35. Returning light on segment 34impinges on beam splitter 32. Part of the light is reflected alongsegment 30 and is lost to the system. The remaining light is passed bybeam splitter 32 to beam segment 47, which is focused by lens 48 ontodetector 18.

The foregoing explains the system configuration to test fiber 10. Totest fiber 11 instead, it requires only the removal of rotator 36 fromthe system, as will now be shown. This removal may be as simple asmerely shifting rotator 36 to one side, out of the path of beam segment34. Alternately, the rotator 36 may itself be rotated so its optic axis60 is aligned parallel or perpendicular to the input polarization 35.These methods have the same result which is to cause no rotation ofpolarization.

In FIG. 2, the components and segments bear the same identifyingnumerals as in FIG. 1 to the extent they are common to both. In thisconfiguration all forward optical activity is the same until beamsegment 34. Now, with the rotator removed or rotated, the plane ofpolarization remains vertical when it impinges on reflector 38. Beamsegment 39 is also vertically polarized as shown by arrows 49 (ratherthan horizontal as it was in FIG. 1). Light in segment 39 impinges onpolarizing beam splitter 41, which as before reflects verticallypolarized light, and passes horizontally polarized light. However, nowthere is no horizontally polarized light to pass, so no light passes tosegment 42. Instead, substantially all of the light is reflected alongsegment 50, vertically polarized as shown by arrows 51. This light isfocused by lens 52 onto fiber 11.

If the fiber has no discontinuities, this light is reflected by dichroicreflector 13, and returns to segment 50 randomly polarized. Polarizingbeam splitter 41 will reflect vertically polarized light along segment39 as shown by arrows 49. It will pass from the system horizontallypolarized light along segment 53.

Light from segment 39 is reflected by reflector 38 along segment 34, andpart of it is passed by beam splitter 32 to segment 47, lens 48 anddetector 18. The reflected portion is lost to the system along segment30.

From the foregoing it will be appreciated that light from the same laserdiode has tested the continuity of two different fibers, the selectionbeing accomplished by a simple mechanical motion of a single opticalelement. The optical components are all well-known, and their selectionis merely one of convenience and function.

Circuitry to determine whether the light received by the detector is oris not respective to continuity forms no part of the invention. Theinvention comprises the optical arrangement which enables the selectionof one fiber to be tested or the other.

This invention provides a reliable and simple testing device, useful totest a plurality of fibers without requiring separate test system foreach fiber, or of discrimination techniques.

This invention is not to be limited by the embodiment shown in thedrawings and described in the description, which is given by way ofexample and not of limitation, but only in accordance with the scope ofthe appended claims.

I claim:
 1. An optical system for individually testing each one of aplurality of optical fibers for continuity, each said fiber having aproximal end and a distal end with a dichroic reflector at each distalend reflective of wavelengths respective to a source of testing lightand transmissive to other wavelengths, said system comprising:a sourceof testing light; a first polarizing element adapted to reflectimpinging light of a selected polarization along a segment, and to passall other light out of the system, a non-polarizing first beam splitterwith one axis normal to that of a source of said testing light and oneaxis coincident therewith, a polarizing second beam splitter having theproperty of transmitting only light polarized in one plane andreflecting only light polarized in a plane that forms a 90 degreedihedral angle therewith, and a polarization rotator half-wave plate inthe optical path between the first beam splitter and the second beamsplitter, said rotator being insertable into and removable from theoptical path to determine which plane of polarization will representlight to be presented from said second polarizing beam splitter to aselected one of said fibers.
 2. An optical system according to claim 1in which a reflector is disposed between said first beam splitter andsaid second polarizing beam splitter to enable said source and saiddetector to be offset from a beam segment incident on second polarizingbeam splitter.
 3. An optical system for individually testing each one ofa plurality of optical fibers for continuity, each said fiber having aproximal end and a distal end with a dichroic reflector at each distalend reflective of wavelengths respective to a source of testing lightand transmissive to other wavelengths, said system comprising:a sourceof testing light; a first polarizing element adapted to reflectimpinging light of a selected polarization along a segment, and to passall other light out of the system, a non-polarizing first beam splitterwith one axis normal to that of a source of said testing light and oneaxis coincident therewith, a polarizing second beam splitter having theproperty of transmitting only light polarized in one plane andreflecting only light polarized in a plane that forms a 90 degreedihedral angle therewith, and a polarization rotator half-wave plate inthe optical path between the first beam splitter and the second beamsplitter, said rotator being rotatable in the optical path to determinewhich plane of polarization will represent light to be presented fromsaid second polarizing beam splitter to a selected one of said fibers 4.An optical system for individually testing each one of a plurality ofoptical fibers for continuity, each said fiber having a proximal end anda distal end with a dichroic reflector at each distal end reflective ofwavelengths respective to a source of testing light and transmissive toother wavelengths, said system comprising:a source of polarize testinglight; a non-polarizing first beam splitter with one axis normal to thatof a source of said testing light and one axis coincident therewith, apolarizing second beam splitter having the property of transmitting onlylight polarized in one plane and reflecting only light polarized in aplane that forms a 90 degree dihedral angle therewith, and apolarization rotator half-wave plate in the optical path between thefirst beam splitter and the second beam splitter, said rotator beinginsertable into and removable from the optical path to determine whichplane of polarization will represent light to be presented from saidsecond polarizing beam splitter to a selected one of said fibers.